Both the CDU and the CSU were established after World War II and share a perspective based on Christian democracy and conservatism, and hold the dominant centre-right position in the German political spectrum. The CSU is usually considered the de facto successor of the Weimar Republic era Bavarian People's Party, which itself broke away from the all-German Catholic Centre Party after World War I. The CDU's foundation however was the result of a major re-organisation of the centre-right political camp compared to the Weimar Republic. Though the CDU was largely built as the de facto successor the Catholic Centre Party, it successfully opened out to non-Catholic Christians, many of them affiliated with the German People's Party until 1933, and successfully asserted itself as the only major conservative party (outside of Bavaria) against initial competition from other Catholic, Protestant or nationalist conservative parties during the early years of the Federal Republic. However, the CDU was and still is significantly stronger in Catholic-dominated areas than in Protestant areas of Germany.

On their political stances, the CDU and the CSU usually only differ slightly. The CSU is usually considered a bit more socially conservative (especially on family issues, e.g. the CSU favors providing infants' parents with compensation if they intend not to use the public day nursery system to work while the CDU favors public funding of day nurseries), Eurosceptic (in 1998, then-Chancellor Helmut Kohl of the CDU had to pressure the CSU intensely not to veto the introduction of the Euro in Germany) and regionalist (CSU politicians often make their mark as self-declared defenders of Bavaria's state rights and cultural independence from federal bureaucrats, even in times of conservative federal governments). While both parties officially identify themselves as non-denominationalChristian, the Catholic influence on the CSU is far stronger than that on the CDU, since Bavaria is predominantly Catholic, while Christians in Germany as a whole are approximately equally balanced between Catholics and Protestants.

The differences between the CDU and the somewhat more socially conservative CSU have sometimes led to conflicts in the past. These tensions climaxed during the 1970s, when Helmut Kohl became CDU chairman in 1972, then considered a moderate or even progressive and also a personal foe of the right-wing then-CSU chairman Franz Josef Strauss, who had held that office since 1961.

After the CDU/CSU narrowly lost the West German federal election of 1976 which had seen Kohl as the common chancellor candidate of the two parties, the CSU's future Bundestag representatives met on November 19, 1976 at a closed meeting at Wildbad Kreuth, at a Hanns Seidel Foundation compound, which is CSU's educational foundation. With a vote of 30–18 and one abstention (and one invalid vote), the CSU deputies decided to separate from their common faction with the CDU deputies in the Bundestag. The decision had been initiated by CSU chairman Strauss, then himself a Bundestag deputy.

The official reasons were to create a more effective opposition (the CDU would approach moderate conservatives, while the CSU would approach the right) and gain more speaking time in parliament.

As a party chairman, Strauss also announced that the CSU as a party would also terminate its self-restriction to Bavaria and foster the foundation of local CSU associations outside of the party's home state, running in all future German federal and state election against the CDU on a distinctly more conservative platform than the CDU's. Strauss therefore coined the term Vierte Partei (fourth party, after the CDU, the Social Democrats and the Free Democrats). This term was technically misleading, since the CSU had always been a distinct party from the CDU, therefore four parties had already been represented during previous Bundestag terms.

On December 12, 1976, the vote was rescinded after the CDU had threatened in turn to form local associations within Bavaria and to run in Bavarian elections against the CSU.

Parliamentary chairpersons of the CDU/CSU group in the Bundestag[edit]